1. Field of the Invention
This invention relates to quality assurance methods, apparatus, and articles of manufacture used for quality assurance of surface treatment of a substrate surface such as peening and, more particularly, to using in plane coupon deflection or bending of a metal coupon or strip for quality assurance of laser shock peening processes.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser shock peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or un-coated surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP'D to create the beneficial compressive residual stresses.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following co-pending U.S. patent application Ser. No. 08/362,362 entitled "ON THE FLY LASER SHOCK PEENING", filed Dec. 22, 1994; and U.S. Pat. No. 5,591,009, entitled "Laser shock peened gas turbine engine fan blade edges"; U.S. Pat. No.5,569,018, entitled "Technique to prevent or divert cracks"; U.S. Pat. No. 5,531,570, entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges"; U.S. Pat. No. 5,492,447, entitled "Laser shock peened rotor components for turbomachinery"; 5,674,329, entitled "Adhesive tape covered laser shock peening"; and U.S. Pat. No. 5,674,328, entitled "Dry tape covered laser shock peening", all of which are assigned to the present Assignee. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock peening.
Laser shock peening processes have been developed to simultaneously LSP pressure and suction sides of leading and trailing edges of fan and compressor airfoils and blades as disclosed in U.S. Pat. No. 5,591,009 entitled "Laser shock peened gas turbine engine fan blade edges" and U.S. Pat. No. 5,531,570 entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges". Almen strips have been used in conventional methods to assess the quality of shot peening processes. Though relatively inexpensive and easy to use, such a process is not suitable because it relies on out of plane bending of the strip. Out of plane bending essentially would not occur with a dual sided and/or simultaneous LSP process because there would be substantially equal compressive residual stresses imparted to both sides of such an Almen strip.
Typically, a flat Almen strip is secured in a holder which constrains the strip to prevent the strip from bending. The strip is subjected to a shot peening regimen after which the strip is removed from the holder. Stresses induced in the strip by the surface shot peening cause the strip to curve out of the plane of the flat strip when removed from the constraints of the holder. Hence, the terms out of plane and in plane as used herein. The curved strip is then typically placed in a peening-intensity gage which measures the amount of out of plane deflection or bending of the strip also referred to as the curvature of the strip. This deflection gives a qualitative indication of the amount of residual stress that is in the Almen strip, though there is no direct quantitative correlation to the residual stress measurement from the measured deflection.
Conventional high cycle fatigue (HCF) testing of blades which are LSP'd and notched before testing has been tried as a quality assurance technique. Measurement of the diameter and volume of a single LSP spot on a flat coupon by optical interferometry has also been tried for QA purposes. Both of these methods are fairly expensive and time consuming to carry out and significantly slows production and the process of qualifying LSP'd components. An improved quality assurance apparatus and method of measurement and control of LSP is required which is inexpensive, accurate, and quick. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. Therefore, any techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.